It is well known that dyes in their reduced leuco form can provide the basis of color image forming systems. The leuco dyes may initially be relatively colorless, but can return to a colored form when oxidized, e.g., by air under acidic conditions. Examples of leuco dyes used in color image forming systems include triarylmethanes, xanthenes, styryl dyes, and azine dyes, such as, for example, phenoxazines, phenothiazines, and phenazines.
It is also known that the leuco form of a given dye can often revert spontaneously to the colored form of the dye, at the time of making the leuco form, unless the structure of the dye is altered to promote stability.
In preparing transfer sheets and carbonless papers, where the leuco form of methylene blue [3,7-bis(dimethylamino)phenothiazine] has frequently been used, stabilization has been brought about by acylation at the 10-position. For example, in U.S. Pat. No. 2,646,367 benzoylated leuco methylene blue is disclosed for use in a carbonless paper system.
U.S. Pat. No. 2,783,227 discloses that benzoylated leuco methylene blue of U.S. Pat. No. 2,646,367, when used in carbonless papers, is not very light-stable, i.e. it decomposes to the blue-green color on exposure to light. Substitution of the 1-naphthoyl group or the 2-naphthoyl group in the 10-position provides more light-stable derivatives of leucomethylene blue than does benzoylation in the 10-position.
U.S. Pat. No. 2,783,228 discloses that nitrobenzoylated leuco methylene blue is more light-stable than the benzoylated leuco methylene blue.
The foregoing patents concern acylated leuco methylene blue and explicitly mention substitution with naphthoyl groups. They do not involve acylation in the 3,7-positions of phenothiazine; they involve acylation only of the ring nitrogen. Furthermore, they make no distinction between the usefulness of 1-naphthoyl groups and 2-naphthoyl groups.
German Offenlegungsschrift No. 2154659 discloses leuco dyes for use in carbonless papers, which dyes are derived from dye nuclei of phenothiazines, phenoxazines, and phenazines by acylating the ring nitrogen with a substituent of the form V--Q--CO-- where Q is O or NH and V is an aromatic or aliphatic hydrocarbon. Nucleus substituents on the 3-position are secondary or tertiary amino groups, and substituents on the 7-position are hydrogen, hydroxyl, amino, or substituted amino groups. Only the ring nitrogen is indicated as being acylated. German Offenlegungsschrift No. 2154660 discloses phenoxazines acylated on the ring nitrogen with a simple acyl radical (preferably benzoyl). Substituents on the 3- and 7-positions can be chosen from hydroxyl, amino, or substituted amino groups, and one of the substituents on the 3- and 7-positions can be hydrogen. The foregoing German patents concern acylated azine leuco dyes including phenazines. However, these patents are not concerned with acylation of the unsubstituted amine groups in the 3- and 7-positions.
British Patent Specification No. 1,271,289 deals with stabilized leuco dyes derived from water soluble dyes containing azine or oxazine rings in the chromophore. It discloses that the stabilized leuco dyes of the invention have an acyl radical attached to a tertiary nitrogen atom in the reduced ring system, and that the acyl radical can be naphthoyl. However, this patent, like the foregoing German patents, ignores the possibility of acylating the unsubstituted amine groups in the 3- and 7-positions.
Apart from their utility in transfer sheets and carbonless papers, acylated leuco forms of dyes, including phenothiazines and phenoxazines, have been found to be useful in thermographic imaging systems.
Japanese patent application No. JA49-60264 dated May 30, 1974, and published as an examined application as No. JA52-23806 on June 27, 1977 discloses a bilayer thermographic color imaging system in which one layer comprises a colorless leuco dye with an inorganic nitrate under acid conditions. The leuco dyes are of the phenothiazine and phenoxazine classes, acylated in the 10-position to stabilize them. Only benzoyl, substituted benzoyl, and alkoxy carbonyl groups are used, and the 3,7-amino substituents are limited to tertiary amines.
Japanese patent application No. JA49-67026 dated June 14, 1974, and published as an examined application as No. JA52-25330 on July 7, 1977 discloses leuco dyes having structural formulae directed to phenoxazine and phenothiazine leuco dyes with acylation of the 10-position nitrogen. Although only acyl groups from fatty acids are included, naphthoyl leuco methylene blue is listed therein.
Current thermographic compositions employing combinations of leuco dyes and nitrate salts are exemplified by assignee's copending applications U.S. application Ser. No. 199,444, filed Oct. 22, 1980, which discloses a composition containing leuco dyes, oxidizable to a colored form, in reactive association with a nitrate salt in a binder; U.S. application Ser. No. 218,559, filed Dec. 22, 1980, which discloses a composition containing a minimum of two leuco dyes in reactive association with a nitrate salt in a binder, said composition capable of providing a black image, and U.S. application Ser. No. 352,053, filed Feb. 24, 1982, which discloses a thermographic leuco dye/nitrate salt composition stabilized with a synergistic combination of antioxidants, e.g., phenidone plus catechol. Amongst the preferred classes of dyes for the compositions of U.S. application Ser. No. 218,559 and U.S. application Ser. No. 352,053 are N-acyl oxazine dyes, such as, for example, benzoylated leuco phenoxazine and N-acyl thiazine dyes, such as, for example, benzoylated leuco methylene blue.
In previous systems of thermography based on combinations of leuco dyes and nitrate salts, at least two approaches have been applied to address the problem of stability to both thermal and actinic effects. The thermal effect, with respect to thermographic processes for preparing transparencies for overhead projection, refers to the problem of attaining the highest possible thermal speed difference between temperatures ranging from 100.degree. C. to 200.degree. C., which range is typical of the image exposure, and those temperatures ranging from 25.degree. C. to 60.degree. C., which range is typical of the projector stage after prolonged operation. The actinic effect refers to problems associated with the high light intensity and elevated temperature to which the finished transparency is subjected on the projector stage.
One approach involves adding to the imaging composition stabilizing compounds, which, in most cases, are normally mild antioxidants, such as, for example, phenidone or ascorbic acid. A second approach involves altering the structure of the leuco dye. Although the first approach has sometimes been successful, it often leads to loss of sensitivity at imaging temperatures. Therefore, the second approach is preferable.
Although the previously mentioned, structurally altered leuco dyes are suitable for providing green and blue images, dyes for providing red and magenta images are so unstable that they require the addition of stabilizing compounds. Even in the presence of stabilizing compounds, their stability is often marginal.
Examination of the prior art indicates that leuco dyes with a phenazine nucleus will provide image colors in the red region. These leuco dyes have been found to exhibit good stability when acylated in the 10-position. Further analysis indicates that a given leuco phenazine with no acyl substituent in the 10-position is not only unstable to light, but is even much more unstable to light than the analagous leuco phenothiazine which contains no acyl substituent in the 10-position. Although benzoylation in the 10-position improves the light-stability of both leuco phenazines and leuco phenothiazines, such benzoylated leuco phenothiazines are useful compounds for thermographic imaging compositions, while the analagous benzoylated leuco phenazines are still far too unstable to be useful for similar types of thermographic imaging compositions.